Polymers manufactured through polycondensation such as polyester typified by polyethylene terephthalate (hereinafter abbreviated as PET) and polybutylene terephthalate (hereinafter abbreviated as PBT), and polyamide typified by nylon 6 (hereinafter abbreviated as N6) and nylon 66 (hereinafter abbreviated as N66) have been preferably used in such applications as clothes and industrial materials, because of the favorable mechanical properties and heat resistance of these fibers. Polymers manufactured through addition polymerization typified by polyethylene (hereinafter abbreviated as PE) and polypropylene (hereinafter abbreviated as PP), in contrast, have been preferably used mainly in industrial applications, because of the favorable mechanical properties, resistance to chemicals and lightness of these fibers.
The polyester fiber and the polyimide fiber, in particular, have been used in the applications for clothes and therefore have been subjected to vigorous researches for not only to modify the polymer but also to improve the properties by controlling the cross sectional shape of the fiber or using an extremely fine fiber. One of such attempts resulted in ultrafine polyester fibers made by using an islands-in-sea multi-component fiber, that was used in an epoch making new product of synthetic leather having the touch of suede. These ultrafine fibers have been applied to the manufacture of ordinary clothes, and are used in the development of clothes that have excellent hands like peach skin which can never be obtained with ordinary fibers. The ultrafine fibers, those have found applications not only for clothes but also for livingwares such as wiping cloth and industrial materials, have secured a position of its own in the area of synthetic fibers today.
Recently, in particular, applications of the ultrafine fibers have been expanded to texturing cloth for the surface of a computer hard disk as described in Japanese Unexamined Patent Publication No. 2001-1252, and medical supplies such as cell adsorbing material as described in Japanese Unexamined Patent Publication No. 2002-172163.
Accordingly, there has been demand for further finer fibers in order to make a synthetic leather of higher quality and clothes of excellent feeling. In the meantime, to increase the storage capacity of a hard disk with increased recording density, it is necessary to make the surface of the hard disk smoother from the mean surface roughness of 1 nm or more at the present to 0.5 nm or less. For this purpose, nanofibers having further decreased thickness have been required to make a texturing cloth for texturing the hard disk surface.
In medical applications, too, nanofibers having the same size as the fibers that constitute living organs have been in demand in order to improve the affinity with the living cells.
However, the present islands-in-sea multi-component spinning technology has a limitation of 0.04 dtex (equivalent diameter 2 μm) for improving the single fiber fineness, which cannot fully meet the needs for the nanofibers. While methods for making ultrafine fibers from polymer blend fibers are disclosed in Japanese Unexamined Patent Publication No. 3-113082 and in Japanese Unexamined Patent Publication No. 6-272114, a single fiber fineness that can be achieved by these technologies is 0.001 dtex (equivalent diameter 0.4 μm) at the best, which also cannot fully meet the needs for the nanofibers.
A method for making an ultrafine fiber from polymer blend fibers using a static mixer is disclosed in U.S. Pat. No. 4,686,074. The ultrafine fibers manufactured by this technology were also not fine enough to meet the needs for the nanofibers.
Meanwhile a technology called the electrospinning has been in spotlight as a promising technology that can manufacture ultrafine fibers. The electrospinning is a process in which a polymer is dissolved in an electrolysis solution and is extruded through a spinneret while applying a high voltage in a range from several thousands of volts to thirty kilovolts to the polymer solution, so as to generate a high speed jet of the polymer solution that subsequently deflects and expands, thereby producing the ultrafine fibers. This technology may produce, depending on the circumstance, yarns having a single fiber fineness on the order of 10−5 dtex (equivalent single fiber diameter several tens of nanometers), that is one hundredth or less in fineness and one tenth or less in diameter of the yarn produced by the conventional polymer blending technology. While this technology is mainly applied to bio-polymer such as collagen and water-soluble polymer, electrospinning may also be applied to thermoplastic polymer that is dissolved in an organic solvent. However, as is pointed out in Polymer, vol. 40, 4585 (1999), the strings that constitute the ultrafine fibers are often connected by beads (about 0.5 μm in diameter) that is formed from a stagnant polymer drop, thus resulting in a large spread of single fiber fineness values in an aggregate of ultrafine fibers. Although attempts have been made to suppress the generation of the beads so as to generate a fiber of uniform diameter, there still remains a significant spread of single fiber fineness values (Polymer, Vol. 43, 4403 (2002)). Also because the form of the aggregate of fibers obtained by the electrospinning is limited to nonwoven fabric and the aggregate of fibers obtained is not oriented and not crystallized, in many cases, having far less strength compared to ordinary fibrous articles, there has been a limitation to the application of the technology. Moreover, there have been such problems that sizes of the fibrous articles manufactured by the electrospinning process are limited to about 100 cm2 at the most, and productivity is as low as several grams per hour at the best that is far lower than with the ordinary melt spinning processes. Furthermore, requirement for the application of a high voltage and the tendency of the organic solvent and the ultrafine fibers to be suspended in air were additional problems.
An atypical method for manufacturing nanofibers is disclosed in Science, Vol. 285, 2113 (1999), according to which a polymerization catalyst is supported on a meso-porous silica so as to polymerize PE thereon, thereby to produce PE nanofiber chips measuring 30 to 50 nm (equivalent to 5×10−6 dtex to 2×10−5 dtex) in diameter. However, what can be obtained with this method is mere wad-like aggregate of nanofibers, which makes it impossible to draw a fiber therefrom. Also the polymer that can be processed with this method is limited to PE manufactured through addition polymerization. Polymers manufactured through polycondensation such as polyester and polyamide require dehydration in the process of polymerization, and there is a fundamental difficulty for applying the method to these fibers. Thus there has been a significant hurdle for practical application of the nanofibers obtained by this method.